The Universal Serial Bus (‘USB’) system is a fast, bi-directional, isochronous, low cost, dynamically attachable serial interface. The expression ‘Universal Serial Bus transmitter’ is used herein to mean a transmitter that fulfils the general functional requirements of the Universal Serial Bus standard, whether or not it meets the detailed manufacturing and operational tolerances of the USB standard.
The USB standard was developed to define an external expansion bus which makes adding peripherals to a personal computer (‘PC’) or similar data processing device as easy as hooking up a telephone to a wall-jack. The standard is published on the Internet at the site http://www.usb.org/developers/docs. The standard's driving goals were ease-of-use and low cost. These were enabled with an external expansion architecture, which highlights:                PC host controller hardware and software,        robust connectors and cable assemblies,        peripheral friendly master-slave protocols,        expandable through multi-port hubs.        
The USB is a cable bus that supports data exchange between a host computer and a wide range of simultaneously accessible peripherals. The attached peripherals share USB bandwidth through a host scheduled token based protocol. The bus allows peripherals to be attached, configured, used, and detached while the host and other peripherals are in operation. This is referred to as dynamic (or hot) attachment and removal.
The Universal Serial Bus connects USB devices with the USB host. The USB physical interconnect is a tiered star topology. A hub is at the centre of each star Each wire segment is a point-to-point connection between the host and a hub or function, or a hub connected to another hub or function. There is only one host in any USB system. The USB interface to the host computer system is referred to as the host controller. The host controller may be implemented in a combination of hardware, firmware, or software.
The USB supports transfer rates up to 12 Mbs for the real-time data of voice, audio, and compressed video, with extension planned to higher speeds. Signal integrity is enhanced by using differential drivers, receivers and shielding. Features provided include: cyclic redundancy check (‘CRC’) protection over control and data fields, error handling/fault recovery mechanisms built into the protocol, self-recovery in protocol, using time-outs for lost or broken packets, and support for identification of faulty devices. Two modes of operation are provided by the standard: low speed, for Interactive Devices at 10-100 Kb/s, and medium speed at 500 Kb/s to 12 Mb/s, for Phone, Audio, Compressed Video.
A root hub is integrated within the host system to provide one or more attachment points. USB transfers signal and power over a four wire cable shown schematically in FIG. 1 of the accompanying drawings. The signaling occurs over two wires D+ and D− and point-to-point segments. The signals on each segment are differentially driven into a cable of 90Ω intrinsic impedance. The differential receiver features input sensitivity of at least 200 mV and sufficient common mode rejection. A clock is transmitted encoded along with the differential data. The clock encoding scheme is NRZI with bit stuffing to ensure adequate transitions. A SYNC field precedes each packet to allow the receiver(s) to synchronize their bit recovery clocks. The cable also carries VBus and GND wires on each segment to deliver power to devices.
The USB uses a differential output driver to drive the USB data signal onto the USB cable. The static output swing of the driver in its low state must be below the VOL of 0.3 V with a 1.5 kΩ load to 3.6 V and in its high state must be above the VOH of 2.8 V with a 15 kΩ load to ground. The output swings between the differential high and low state must be well balanced to minimize signal skew. Slew rate control on the driver is required to minimize the radiated noise and cross talk. The driver's outputs must support three-state operation to achieve bi-directional half duplex operation. High impedance is also required to isolate the port from downstream devices that are being hot inserted or which are connected but powered down.
The signals transmitted on the two signal wires, referred to as ‘USBP’ and ‘USBM’ need to be synchronized and it is important that the falling edge of one signal coincide with the rising edge of the other and, in particular, it is important to control the cross-over level, that is to say the voltage at which the signals on the two wires have the same value, and the jitter, that is to say the variations for successive signals in the time at which the cross-over point occurs.
Patent specification U.S. Pat. No. 5,912,569 discloses a transmitter in which one of the output signals is delayed relative to the other in order to correct the crossover level.